<%= start_form_tag ({:controller=>"slideshows" ,:action => 'update_image'} , {:multipart => true}) %>
<p>Choose a picture to upload (jpg)</p>
<p><%= file_column_field 'slideshow_image', 'image_url', {:size => 20} %></p>
<p><%= hidden_field 'slideshow_image', 'slideshow_id' , {:value => @slideshow.id} %></p>

  <p><%= submit_tag 'Upload the file' %></p>
<%= end_form_tag %>